1. Field of the invention
The present invention relates to an exhaust gas purification device for an engine. More specifically, the present invention relates to a device which is capable of effectively removing an NO.sub.x component from exhaust gas of the engine.
2. Description of the Related Art
An exhaust gas purification device used for engines mainly operated with a lean air-fuel ratio (such as lean burn gasoline engines) to remove an NO.sub.x (nitrogen oxide) components in the exhaust gas is disclosed in WO93-07363. The device utilizes an NO.sub.x absorbent which can absorb an NO.sub.x component in the exhaust gas when the air-fuel ratio of the exhaust gas is lean, and can release absorbed NO.sub.x when the oxygen concentration in the exhaust gas becomes low. In the device disclosed in WO93-07363, the NO.sub.x absorbent is disposed in the exhaust passage of a lean burn engine. Since the air-fuel ratio of the exhaust gas in the most part, lean in the lean burn engine, the NO.sub.x absorbent usually absorbs the NO.sub.x components in the exhaust gas, hence the NO.sub.x component is removed from the exhaust gas. When the amount of the NO.sub.x absorbed in the NO.sub.x absorbent increases, the air-fuel ratio of the engine is lowered to make the air-fuel ratio of the exhaust gas rich. When the air-fuel ratio of the exhaust gas becomes rich, the oxygen concentration in the exhaust gas becomes low and, at the same time, the concentration of HC and CO components in the exhaust gas becomes higher, therefore, NO.sub.x absorbed in the NO.sub.x absorbent is released from the absorbent and reduced to N.sub.2 by the HC and CO components in the exhaust gas.
By repeating above procedure during engine operation, the NO.sub.x component in the exhaust gas is almost completely removed by the device in WO93-07363.
As stated above, the NO.sub.x absorbent can absorb NO.sub.x in the exhaust gas when the air-fuel ratio of the exhaust gas is lean. However, it is necessary that an oxygen component exists in the exhaust gas when the NO.sub.x absorbent absorbs NO.sub.x in the exhaust gas. Therefore, when the air-fuel ratio of the exhaust gas is lower than a stoichiometric air-fuel ratio, i.e., when the air-fuel ratio of the exhaust gas is rich, the NO.sub.x absorbent cannot absorb the NO.sub.x in the exhaust gas since the concentration of the oxygen in the exhaust gas is too low.
Therefore, when the exhaust gas purification device of WO93-07363 is applied to engines mainly operated near the stoichiometric air-fuel ratio, such as the engines in which air-fuel ratio oscillates between a rich air-fuel ratio and a lean air-fuel ratio about the stoichiometric air-fuel ratio, the NO.sub.x absorbent cannot absorb NO.sub.x in the exhaust gas when the air-fuel ratio of the engine is rich. Therefore, the NO.sub.x absorbent is not effective as a means for purifying the exhaust gas of such engines.
As the exhaust gas purification device for the engines which are operated near the stoichiometric air-fuel ratio, a three-way reducing and oxidizing catalyst is widely used. The three-way reducing and oxidizing catalyst can remove three pollutants in the exhaust gas, i.e., HC, CO and NO.sub.x simultaneously when the air-fuel ratio of the exhaust gas is near the stoichiometric air-fuel ratio. However, the three-way reducing and oxidizing catalyst can remove these three pollutants in the exhaust gas only when the air-fuel ratio of the exhaust gas is in narrow range around the stoichiometric air-fuel ratio, and the efficiency for removing NO.sub.x in the exhaust gas decreases rapidly as the air-fuel ratio of the exhaust gas becomes lean.
Therefore, when applied to the engines in which the air-fuel ratio oscillates periodically between rich and lean air-fuel ratio about the stoichiometric air-fuel ratio as the center of oscillation, the total capability of three-way catalyst for removing NO.sub.x in the exhaust gas becomes low, because of lower efficiency for removing NO.sub.x during the period in which the air-fuel ratio of the exhaust gas is on the lean air-fuel ratio side.
To compensate for the deficiency of the three-way reducing and oxidizing catalyst in the lean air-fuel ratio region, it is possible to dispose more than two three-way catalysts in series. However even when arranged in such a manner, the ability of the three-way catalysts for purifying NO.sub.x in total does not increase remarkably. The ability of the three-way catalyst for reducing NO.sub.x falls rapidly as the concentration of NO.sub.x in the exhaust gas decreases, since the possibility of the NO.sub.x molecules being reduced on the catalyst becomes lower as the concentration of the NO.sub.x in the exhaust gas becomes lower.
When more than two catalysts are disposed in the exhaust passage in series,the concentration of NO.sub.x in the exhaust gas flowing into the catalysts disposed at downstream side becomes low, since the large part of the NO.sub.x is already removed by the catalyst disposed at upstream side of the exhaust passage. Therefore, the total capability of the catalysts for removing NO.sub.x in the exhaust gas as a total becomes almost the same as the capability of the first catalyst. For example, if two three-way catalysts are disposed in the exhaust passage, 95 percent of the NO.sub.x in the exhaust gas is removed by the first (upstream) catalyst, and due to lower concentration of the NO.sub.x component in the exhaust gas flowing into the second (downstream) catalyst, the ability of the downstream catalyst becomes very low, and total ability of two catalysts becomes almost same value of the ability of the upstream catalyst (e.g., approximately 96 percents).
Therefore, in case of engines which air-fuel ratio oscillates between a rich air-fuel ratio and a lean air-fuel ratio, it is very difficult to effectively remove NO.sub.x in the exhaust gas.